1. Filed of the Invention
The present invention relates to the production of a high porosity semiconductor thin film via a plasma-enhanced chemical-vapor deposition system. More specifically, the present invention relates to a unique high density plasma approach which uses simultaneous plasma deposition and etching to obtain high porosity semiconductor thin film.
2. Description of Related Art
There is a great deal of interest in porous silicon structures. The reasons for this are two-fold. First, these porous films can be used in a variety of applications including MEMS (microelectro-mechanical devices), interconnect dielectic, micro-sensor, cell and molecule immobilization, and micro-fluidic applications. Secondly, the material is very compatible with Si microelectronics. There are various approaches to producing porous silicon materials. The technique attracting the most attention today for the fabrication of porous silicon is based on the use of wet chemical solutions and the electrochemical technique of anodization (R. C. Anderson, R. C. Muller, and C. W. Tobias, Journal of Microelectromechanical System, vol. 3, (1994), 10). Heretofore, this technique has yielded the best level of porosity among known approaches. The starting material for this wet etched material is either conventional silicon wafers or thin film Si produced by some deposition process such as low pressure chemical vapor deposition (LPCVD) or plasma enhanced chemical vapor deposition (PECVD). In the electrochemical wet etching process the sample is exposed to a wet solution and a current is passed through a contact to the etching sample, through the etching sample, through the solution (e.g., a mixture of hydrofluoric acid, water and ethanol), and through an electrode contacting the solution (the cathode; e.g., platinum). This current causes the xe2x80x9cpittingxe2x80x9d or etching of the Si producing a porous network structure.
In the electrochemical (anodic) etching process the structure (e.g., pore size and spacing) and the porous-Si layer thickness are controllable by the resistivity of the silicon itself (magnitude and type), current density, applied potential, electrolyte composition, application of light, temperature, and exposure time. For sufficiently long exposures and for sufficiently thick starting material, this electrochemical etching process can be continued to the point where nanoscale structure (i.e., features of the order of nanometers) is obtained. The silicon features are a continuous single crystal when the sample is etched from a single crystal wafer or large grain polycrystalline silicon when the sample is etched from a deposited film. All these conventional (electrochemically etched) porous silicon materials are distinguished by (1) being the result of a wet, electrochemical etching process, (2) requiring a contact on the sample during this wet etching, (3) having generally disconnected pore regions which can be connected after extensive etching, and (4) being the result of a sequential processing first necessitating formation of the silicon and then necessitating subsequent wet etching.
Intense research activity in porous semiconductors has been even further stimulated over the last decade by the discovery of room temperature visible light emission from electrochemically prepared porous Si in 1990 by Canham (L. T. Canham, Appl. Phys. Lett. 57, 1046 (1990)), Soon after Canham""s discovery, further intriguing properties of porous semiconductor materials were also realized, such as gas sensitivity, bio-compatibility and ease of micromachining, etc. (I. Schecter et al., Anal. Chem. 67, 3727 (1995); J. Wei et al. Nature 399, 243 (1999); L. T. Canham et al., Thin Sold Films, 297, 304 (1997); P. Steiner et al., Thin Solid Films 255, 52 (1995)). All demonstrated applications to-date have been based on porous silicon material produced by electrochemically etching a wafer or deposited film of silicon.
The approach to porous silicon in the present invention is to use deposition to grow as-deposited porous films; specifically the porous silicon is a deposited columnar material whose pores are the voids between the columns and clusters of columns. In the present invention, pores (void) regions are reasonably uniform through the thickness of the film and across the film. The present process is unique because it is performed at low temperature, the present inventors have demonstrated that the present invention can be used to control void size and void fraction, the void-column network morphology does not vary over thicknesses of interest, the columns can be polycrystalline material, and a plasma approach can be used to control the interaction between deposition and etching during growth. The present process yields high porosity (of up to approximately 90%), controlled pore size material without any back contacts and anodization-based wet processing. Unlike other deposition processes, the present process is based on high-density plasma deposition-etching interaction and is, therefore, able to give a high degree of controllable porosity (up to 90%), a morphology that does not vary with thickness, and doped or un-doped polycrystalline columns. Also unique to the present invention is its ability to fabricate this porous silicon on various types of substrates including glass, metal foils, insulators, plastic, and semiconductor-containing materials.
The high density plasma (HDP) deposition tool used in this demonstration was an electron cyclotron resonance plasma machine. In particular, our porous silicon was demonstrated by use of a high density plasma tool (e.g., Electron Cyclotron Resonance Plasma Enhanced Chemical Vapor Deposition (ECR-PECVD) tool (PlasmaTherm SLR-770)) using hydrogen diluted silane (H2:SiH4) as the precursor gas at substrate deposition temperatures less than or equal to about 250xc2x0 C. This tool plays off silicon etching and deposition to create a two-dimensional silicon array and analysis has demonstrated that silicon column size is controllable, the spacing between columns is controllable, and morphology does not vary with thickness. Unlike other deposited columnar silicon materials, the column spacing can be maintained as the film grows in thickness and column phase composition can be controllably varied from polycrystalline to amorphous. The resulting void-column network structure is nanoscale in feature size and fully developed after a film thickness in the range of 10-20 nm is established. This enables the direct deposition of high porosity crystalline or amorphous silicon on any substrate and at any thickness greater than about 10 nm, and preferably between 10-20 nm. The porous semiconductor films produced by the present invention may be converted to insulators or metallic compounds through in situ or ex situ processing.
The prior art contains two approaches to porous silicon: (1) wet electrochemical etching of deposited silicon or of silicon wafers to produce a porous silicon with a xe2x80x9ccoral-likexe2x80x9d morphology of polycrystalline or single-crystal silicon xe2x80x9cfingersxe2x80x9d or (2) deposition of silicon to produce a porous material consisting of tapered amorphous silicon rods with a morphology that varies with thickness. The former material has the advantage of a very controllable morphology and porosity and is the subject of a great deal of research and development activity. However, it requires wet chemical etching for its formation. The latter material suffers from only being available in the amorphous phase and of having a morphology that varies with thickness and a porosity that requires subsequent wet etching for control. The porous silicon of this invention requires no wet processing. It has a fully controllable morphology and porosity and can be in the polycrystalline or amorphous phase as desired.
This invention describes porous material deposition at low temperatures. These materials are particularly suitable for deposition on glass or plastic or other substrates requiring low processing temperatures such as substrates containing previously formed sensor, electronic or opto-electronic devices and circuits. Due to the wide, demonstrated porosity range possible for the materials of this invention, they can be used for a number of applications including sensing, airgaps (optical mixing, microfluidics, molecular sorting, low dielectric constant structures, etc.), fixing and electrically contacting molecules and cells, and mass desorption applications. The invention is demonstrated using deposited porous silicon.
The present invention provides a deposited porous film comprising a plurality of perturbations extending therefrom into a void having a porosity of up to about 90%. The plurality of perturbations are disposed substantially perpendicular to a substrate, or in the alternative, to a base layer. The plurality of perturbations are rod-like shaped columns and are semiconducting, and may be polycrystalline, such as a silicon material. The porosity is the result of a continuous void. The perturbations have a height adjustable by the film thickness and are of a diameter from about 1 nm to about 50 nm. More specifically, the columns have a diameter of about 3 nm to about 7 nm. Further, the perturbations are found in clusters having a diameter between about 50 to 500 nm or more.
The present invention also provides a composite structure which comprises a substrate; and a porous film comprising a plurality of perturbations extending therefrom into a void having a porosity of up to 90%, wherein the porous film is disposed on the substrate. The composite structure may further comprise a coating layer, such that the porous film is disposed on the coating layer. The coating layer is at least one of the following materials: insulators such as organic insulators, silicon nitride, and silicon oxide; or at least one of the following active materials: piezoelectrics, ferroelectrics, metals, and semiconductors. The composite structure may further comprise a capping layer, such that the porous film is disposed between the capping layer and the substrate, where the substrate may be coated. The capping layer is at least one of the following materials: insulators such as organic insulators, silicon nitride, and silicon oxide; or at least one of the following active materials: piezoelectrics, ferroelectrics, metals, and semiconductors. The porous film has a thickness greater than about 10 nm, and wherein the film may be varied from polycrystalline to amorphous. This deposited porous film is structured in a two-dimensional periodic array of rod-like perturbations. The porous films can be converted; e.g., oxidized to SiO2, reacted with metals to form silicides or nitrided to Si3N4. The film may also be doped or not doped. The substrate of the composite structure is selected from glass, metals including foils, insulators, plastics, and semiconductor-containing material. The substrate may optionally be coated with an atomic motion barrier, thermal barrier, electrically insulating, or stress-controlling film. An example film is a silicon nitride barrier layer, wherein the thickness of this silicon nitride barrier layer can vary from hundreds of Angstroms or less, to about 5,000 nm. The film may be varied during deposition and throughout the film thickness.
The present invention further provides a method of forming a composite structure comprising a substrate and a porous film. This method comprises the step of depositing a porous film on a substrate via high density plasma deposition. The porous film of this method comprises a plurality of perturbations extending therefrom into a void having a porosity of up to about 90%. The method may further comprise the step of etching the porous film, where the deposition and etching steps preferably occur simultaneously. The etching can be conducted by the presence of corrosive agents in the plasma such as hydrogen, chlorine, fluorine, HCI, HF and their derivative radicals. The above-mentioned high density plasma deposition is conducted in the presence of at least one precursor gas, selected from hydrogen and silicon-containing gas. More specifically, the silicon-containing gas is silane (SiH4 gas). The deposition step is conducted at a temperature of about 250xc2x0 C. or less, and in the presence of a magnetic field at the substrate region in the range between about +800 to xe2x88x92600 Gauss, and in an excitation of microwave power in the range between about 100 Watts to 1200 Watts. The deposition step is also conducted with no impressed voltage between the plasma and substrate. An additional step in the method can be removing the porous layer in the selected regions of porous film by etching, thereby creating an airgap, release, or isolation structure.
More specifically, the present invention provides a method wherein this method is comprised of the steps of preparing a substrate; conditioning the deposition tool surfaces; employing a plasma deposition tool to create ions, radicals, and other excited species; introducing power into a plasma chamber; feeding precursor gases into the plasma deposition tool to ignite the plasma; using controls to further adjust deposition kinetics; and depositing films on a substrate via plasma deposition.
The substrate may further be coated prior to porous film deposition depending on the type of substrate implemented. The substrate is coated using a material selected from those useful for functions such as electrical isolation, planarizing, atomic motion barrier, stress adjustment, and thermal coupling. The substrate may be exposed to surface texturing such as liquid chemical etching or plasma chemical etching.
The plasma deposition tool used in the above method is a high density plasma deposition tool. A specific high density plasma deposition tool for the method is an electron cyclotron resonance tool.
The porosity is controllable with substrate treatments such as coating, plasma power, substrate-region magnetic field, gas composition, deposition temperature, plasma substrate bias, chamber conditioning, process pressure, deposition gases and flow rates. The energy of the plasma species impacting the depositing/etching film is important and must be kept low. This control is assured by a high density plasma system; e.g., in an ECR tool the kinetic energy of the impacting species is expected to be  less than 45 eV.
The present invention also allows for controlling the column phase. For example, at high deposition pressures (e.g., 20 mTorr) the columns will become amorphous instead of polycrystalline in the case of silicon.
Further, the present invention provides for a method of producing a porous film, comprising the steps of preparing the substrates with steps such as barrier coatings; conditioning the chamber; employing a high density plasma deposition tool to create ions, radicals, and other excited species; introducing power into a plasma chamber; feeding precursor gases such as H2 gas into the plasma chamber to ignite the plasma; and SiH4 gas into the deposition chamber; maintaining a temperature for substrate deposition of less than about 250xc2x0 C.; using controls such as substrate-region magnetic field to further adjust deposition kinetics; and depositing films on a substrate.
The microwave power introduced into the ECR plasma chamber is from between about 100 Watts to 1200 Watts, preferably between about 340 Watts to 640 Watts. The microwave power used had a frequency of 2.45 GHz.
The H2 flow rate for the present invention is between about 1 sccm to 500 sccm, preferably between about 10 to 100 sccm. The SiH4 flow rate for the present invention is between about 1 sccm to 300 sccm, preferably between about 2 to 10 sccm.
More specifically, the present invention encompasses a method for producing the above-described film, comprising the steps of: employing a high density plasma deposition tool with a precursor gas, such as hydrogen diluted silane (H2:SiH4); introducing microwave power between about 100 Watts to 1200 Watts into a ECR plasma chamber through a waveguide and a fused quartz window minimizing reflected power by adjusting a tuner; setting up a static magnetic flux density in the vicinity of the substrate of +800 to about xe2x88x92600 Gauss using a DC electromagnet; feeding H2 gas at about 1 sccm to 500 sccm through a gas dispersal ring in the ECR plasma chamber; injecting SiH4 gas at about 1 sccm to 300 sccm through a gas distribution ring about 1.3 cm above the substrate into the deposition chamber; maintaining a temperature for substrate deposition of less than 250xc2x0 C.; and depositing films on a substrate.
The present invention may be used in forming sensors, desorption spectroscopy structures, gas detectors, and airgap (void) structures for plasma display applications, for dielectric applications, for monitoring lateral resistivity and for tube, sorting, and chromatography applications, and for optical structures such as in opto-electronic devices and solar cells for optical impedance applications. These uses typically comprise a composite structure having a substrate; and a porous continuous film comprising polycrystalline or amorphous silicon having a porosity of up to 90% wherein the porous continuous film is disposed on the substrate.